1. Field of the Invention
This invention relates to the manufacture of microcellular polyurethane elastomers having wide applicability in the footwear and automotive industries.
2. Description of the Prior Art
Polyurethane microcellular elastomers are employed in a wide number of end use applications as in the shoe and automotive industries, for example, in the manufacture of shoe soles, bumpers for automotive applications, arm rests of integral skin foams, and the like. In a number of applications a proper balance of physical properties is required in order to achieve optimum results for the intended commercial use.
The manufacture of microcellular urethane elastomers from polyether polyols and prepolymers from 4,4'-diphenylmethane diisocyanate (4,4'-MDI) is well known in the art. The reason such elastomer products are not made by means of the "one-shot" process is due to the fact that the isocyanate reactant, 4,4'-MDI, is not readily miscible in the polyether polyol/glycol chain extender/blowing agent mixtures commonly employed. Moreover, pure 4,4'-MDI has a melting point of approximately 40.degree. C., and at such temperatures the handling of these materials becomes difficult in many of the end use applications illustrated above.
The literature, for example, U.S. Pat. No. 3,644,457, describes the preparation of quasi-prepolymers from various polyether polyols and 4,4'-diphenylmethane diisocyanate having, generally, melting points of from 15.degree.-25.degree. C. In the footwear and automotive industries, it is highly desirable to have the chemical reactants remain liquid during storage and at the average operating temperatures employed during the manufacture of microcellular elastomer products. Once the quasi-prepolymers crystallize, proper action must be taken to reform the original liquid to prevent subsequent dimerization and trimerization, which proceeds rather quickly, with concomitant deterioration of clarity and assay of the product. The recommended technique for melting such crystallized material is slow drum rolling in atmospheric steam. The disadvantages are obvious and well-known in the art. Typical polyisocyanate products that find wide use in the abovesaid fields are the quasi-prepolymers from di(propylene glycol) or dipropylene glycol/tripropylene glycol mixtures and 4,4'-diphenylmethane diisocyanate having equivalent weights of about 180-185. These quasi-prepolymers find extensive use in footwear and automotive applications, but they suffer from the above-mentioned drawbacks inasmuch as their melting points are approximately 17.degree.-25.degree. C., where they exhibit a tendency for dimer and trimer formation.
For automotive applications, mixtures of the above-said quasi-prepolymers (from dipropylene glycol and 4,4'-MDI) with the carbodiimides from 4,4'-MDI, described in U.S. Pat. No. 3,644,457 and German Pat. No. 1,092,007, remain liquid at near 15.degree.-20.degree. C.; however, for footwear applications these mixtures are of little use since integral skin microcellular elastomers therefrom exhibit poor low temperature flex properties.
It is known to prepare quasi-prepolymers from polyether polyols and 4,4'-diphenylmethane diisocyanate that are liquid at relatively low temperature, for example, at 10.degree. C. However, in order for such quasi-prepolymers to remain liquid, the polyether segment of the quasi-prepolymer is at least 50-55% by weight, or more. Such quasi-prepolymers exhibit equivalent weights per NCO group of about 300 or higher. There exists several disadvantages associated with these quasi-prepolymers. In multicomponent systems such as two component systems generally employed in the manufacture of intergal skin microcellular polyurethane elastomer articles, e.g., shoe soles, automotive articles, etc., the high NCO equivalent weights of the polyisocyanate component requires that the "resin" component be a polyol blend comprising high levels of chain extender, e.g., ethylene glycol or 1,4-butanediol. Unfortunately, at such high levels of chain extender which are generally required for the manufacture of medium to high hardness shoe soles and automotive articles (65-95 Shore A hardness) the required resin blend of polyether polyol/chain extender/fluorocarbon blowing agent generally becomes quite incompatible thereby imposing significant processing problems and limitations on such systems. Moreover, bulk shipments and/or storage for even short periods of time are not economically tolerable due to the resulting phase separation of the polyol and the chain extender. Even in use, processing requires appropriate mixing to prevent phase separation. Incompatibility of the system and/or marginal mixing can adversely effect the physical properties of the ultimate elastomer products. As soon as the reactant materials deviate appreciably from the stoichiometric balance between hydroxyl and isocyanate ingredients, the resulting products, for example shoe soles, will crack during wear, making these articles useless in commerce.
An additional drawback of the high NCO equivalent weights, e.g., about 300 and higher, which characterize such quasi-prepolymers is the substantially lower exotherm that multi-component systems are capable of generating during the molding operation since a significant portion of this exotherm has already been dissipated during the formation of the quasi-prepolymers per se. Hence, such systems require the addition of substantial heat during the molding process if one desires short demolding times. The application of substantial external heat (hot molding) process instead of a colder molding process) and the extended mold cycles make such combinations less attractive for economic reasons.